In hollow fiber membrane filtration generally used in the art, a hollow fiber membrane is immersed in water to be treated and separates impurities from the water. Such a hollow fiber membrane has been broadly used in the field of precise filtration for production of sterile water, drinking water, distilled water, and the like, and has recently expanded in application range to sewage/wastewater treatment, solid/liquid separation in septic tanks, suspended solid removal from industrial wastewater, and filtration of stream water, industrial water, swimming pool water, and the like.
Hollow fiber membranes can be classified into a reinforced membrane type, in which a polymer resin film is coated on a tubular fabric or braid, and a single membrane type, in which a hollow fiber membrane is composed only of polymer resin films without using a reinforcing material such as tubular fabrics or braids. The complex film type membrane employs the tubular fabric or braid as a reinforcing material and thus exhibits excellent mechanical properties.
U.S. Pat. No. 6,354,444 discloses a hollow fiber membrane, in which a polymer solution is applied only to an outer surface of a hollow tubular fabric or braid formed of polyethylene terephthalate (PET) fibers or the like and small pores are formed through phase separation to improve tensile strength.
Although such a hollow fiber membrane for water treatment manufactured using a reinforcing material has high tensile strength, it is difficult to obtain uniform coating of the polymer solution on the reinforcing material. As a result, this type of hollow fiber membrane can suffer from defects such as leakage or the like. In addition, upon exposure to impact or pressure for a long duration, the coating can be peeled off or damaged due to deterioration in adhesion between the tubular braid and the high molecular weight layer. Moreover, since it is difficult to coat the polymer solution on the inner side of the hollow fiber due to difficulty for polymers to reach an inner periphery of the hollow fiber upon coating of the polymer solution, contaminants having infiltrated into the hollow fiber membrane upon backwashing can contaminate the tubular braid.